Altering and controlling cell membrane and subcellular region ion permeability has permitted examination of characteristics of cells, tissues, and organisms. Light-driven pumps and channels have been used to silence or enhance cell activity and their use has been proposed for drug screening, therapeutic applications, and for exploring cellular and subcellular function.
Molecular-genetic methods for preparing cells that can be activated (e.g., depolarized) or inactivated (e.g., hyperpolarized) by specific wavelengths of light have been developed (see, for example, Han, X. and E. S. Boyden, 2007, PLoS ONE 2, e299). It has been identified that the light-activated cation channel channelrhodopsin-2 (ChR2), and the light-activated chloride pump halorhodopsin (Halo/NpHR), when transgenically expressed in cell such as neurons, make them sensitive to being activated by blue light, and silenced by yellow light, respectively (Han, X. and E. S. Boyden, 2007, PLoS ONE 2(3): e299; Boyden, E. S., et. al., 2005, Nat Neurosci. 2005 Sep.; 8(9):1263-8. Epub 2005 Aug. 14.). Previously identified light-activated pumps and channels have been restricted to activation by particular wavelengths of light, thus limiting their usefulness.